drm/i915/execlists: Relax CSB force-mmio for VT-d

The original switch to use CSB from the HWSP was plagued by the effect
of read ordering on VT-d; we would read the WRITE pointer from the HWSP
before it had completed writing the CSB contents. The mystery comes down
to the lack of rmb() for correct ordering with respect to the writes
from HW, and with that resolved we can remove the VT-d special casing.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Michał Winiarski <[email protected]>
Cc: Michel Thierry <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Tested-by: Mika Kuoppala <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>

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